In recent years, energy storage devices, especially lithium secondary batteries have been widely used for small-sized electronic devices, such as mobile telephones, notebook-size personal computers and the like, for electric vehicles, as well as for electric power storage. These electronic devices and vehicles may be used in a broad temperature range, for example, at midsummer high temperatures or at frigid low temperatures, and are therefore required to be improved in point of electrochemical characteristics well balanced in a broad temperature range.
Especially for preventing global warming, it is imperative to reduce CO2 emissions, and of environment-responsive vehicles with, as mounted thereon, an electricity storage installation that comprises an energy storage device, such as a lithium secondary battery, a capacitor or the like, early popularization of hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV) and battery electric vehicles (BEV) is desired. Vehicles could take a long travel distance and therefore could be used in regions in a broad temperature range covering from extremely-hot tropical regions to frigid regions. In particular, therefore, it is desired that the electrochemical characteristics of in-car energy storage devices for those vehicles are not worsened even in use thereof in a broad temperature range covering from high temperatures to low temperatures.
In this specification, the term, lithium secondary battery is used as a concept including a so-called lithium ion secondary battery.
A lithium secondary battery is mainly constituted of a positive electrode and a negative electrode containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution containing a lithium salt and a nonaqueous solvent. For the nonaqueous solvent, used are carbonates, such as ethylene carbonate (EC), propylene carbonate (PC), etc.
As the negative electrode, known are metal lithium, and metal compounds (metal elemental substances, oxides, alloys with lithium, etc.) and carbon materials capable of absorbing and releasing lithium. In particular, a lithium secondary battery using a carbon material capable of absorbing and releasing lithium, such as coke, artificial graphite, natural graphite or the like, has been widely put into practical use.
For example, it is known that, in a lithium secondary battery using a highly-crystalline carbon material, such as natural graphite, artificial graphite or the like as the negative electrode material therein, the decomposed products or gas generated through reductive decomposition of the solvent in the nonaqueous electrolytic solution on the surface of the negative electrode during charging detracts from the electrochemical reaction favorable for the battery, therefore worsening the cycle properties of the battery. Deposition of the decomposed products of the nonaqueous solvent interferes with smooth absorption and release of lithium by the negative electrode, and therefore the electrochemical characteristics of the battery in use thereof in a broad temperature range may often tend to worsen.
In addition, it is known that a lithium secondary battery using a lithium metal or its alloy, or a metal elemental substance, such as tin, silicon or the like or its metal oxide as the negative electrode material therein could have a high initial battery capacity but the battery capacity and the battery performance thereof, such as cycle properties may greatly worsen, since the micronized powdering of the material is promoted during cycles thereby bringing about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode of a carbon material. In addition, the micronized powdering of the negative electrode material and the deposition of the decomposed products of the nonaqueous solvent may interfere with smooth absorption and release of lithium by the negative electrode, and therefore the electrochemical characteristics of the battery in use thereof in a broad temperature range may often tend to worsen.
On the other hand, it is known that, in a lithium secondary battery using, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4 or the like as the positive electrode, the nonaqueous solvent in the nonaqueous electrolytic solution locally undergoes partial oxidative decomposition in the interface between the positive electrode material and the nonaqueous electrolytic solution in the charged state and the decomposed products or the gas thereby generated as a result of the partial oxidative decomposition interferes with the electrochemical reaction favorable for the battery, and therefore the electrochemical characteristics of the battery would be thereby also worsened in use in a broad temperature range.
As in the above, the decomposed products and the gas generated through decomposition of the nonaqueous electrolytic solution on the positive electrode or the negative electrode may interfere with the movement of lithium ions or may swell the battery, and the battery performance is thereby worsened. Despite the situation, electronic appliances equipped with lithium secondary batteries therein are offering more and more an increasing range of functions and are being in a stream of further increase in power consumption. With that, the capacity of lithium secondary batteries is being much increased, and the space volume for the nonaqueous electrolytic solution in the battery is decreased by increasing the density of the electrode and by reducing the useless space volume in the battery. Accordingly, the situation is that even decomposition of only a small amount of the nonaqueous electrolytic solution may worsen the electrochemical characteristics of the battery in use thereof in a broad temperature range.
PTL 1 proposes a nonaqueous electrolytic solution containing methyl phenyl carbonate or 4-t-butylphenyl methyl carbonate, and suggests improving overcharge tolerance and continuous charge performance.